The present invention relates to a developing unit included in an image forming apparatus and for forming an image by use of a two-ingredient type developer, i.e., toner and carrier mixture. More particularly, the present invention is concerned with a device for replenishing fresh toner and replacing carrier deteriorated due to aging or a developer including such carrier.
A two-ingredient type developer is extensively used with an electrophotographic copier, facsimile apparatus, laser printer or similar image forming apparatus. This type of developer consists of carrier particles and toner particles depositing on the carrier particles due to frictional charge. Every time a developing unit included in the apparatus effects development, the toner particles are sequentially consumed. As a result, the ratio of the toner to the carrier and therefore the density of the resulting image decreases. It is therefore a common practice to replenish fresh toner to the developer in the same amount as the toner consumed. The carrier particles are each covered with a material for enhancing its frictional charging characteristic. The problem with the carrier particles is that the material covering them comes off due to repeated development, obstructing frictional charging between the carrier particles and the toner particles. The toner with no charge or short charge lowers image quality and flies about to contaminate the inside of the apparatus. Generally, the life of the developer expires when several thousands to several hundreds of thousands of sheets are dealt with.
In light of the above, it has been customary with an image forming apparatus to periodically dismount the developing unit from the apparatus body, evacuate a developing chamber of the deteriorated carrier or developer, and then fill the chamber with a new developer. The developer reached its life is entirely discharged from the developing chamber. The replacement of the developer, i.e., the removal of the developing unit needs an expert serviceman. There is an increasing demand for easy replacement when it comes to a high-speed copier or similar apparatus needing frequent replacement of the developer.
To meet the above demand, Japanese Patent Laid-Open Publication No. 60-107057, for example, proposes an automatic developer replacing system. In the proposed system, a shutter mounted on the bottom of a developing unit is opened in order to discharge the deteriorated developer. Then, a new developer is set in a hopper forming an upper portion of the developing unit. The new developer is automatically fed from the hopper into the developing unit. The system implements the replacement of the developer without the developing unit being dismounted from the apparatus. However, because the deteriorated developer is entirely replaced with a new developer at a time, image quality noticeably differs from the time just before the replacement to the time just after the replacement.
Further, the automatic replacing system requires an additional space for collecting the deteriorated developer, increasing the overall size of the apparatus. Moreover, a tank for the new developer and a container for the used developer are essential and need time- and labor-consuming management. Japanese patent Laid-Open Publication No. 4-118675, for example, teaches an arrangement including a developer replenishing chamber in which toner and carrier are stored independently of each other, and a developer collecting chamber for collecting the deteriorated developer from a developer storing section included in a developing unit. In this arrangement, after the deteriorated developer has been collected, the carrier and toner are sequentially replenished into the developer storing section in this order. While this kind of approach frees the operator from troublesome developer replacement, it also needs a space for the collection of the deteriorated toner. Again, because the developer existing in the developing unit is entirely replaced when its life expires, image quality noticeably differs from the time just before the replacement to the time just after the replacement.
To free image quality from the influence of the life of the developer, the developer may be replaced in a small amount at a time, as disclosed in, e.g., Japanese Patent Laid-Open Publication No. 4-29271. In such a system, when a dry two-ingredient type developer existing in a developing unit is consumed by a preselected amount, it is discharged only in a preselected small amount. In this case, a new developer is fed into the developing unit in substantially the same amount as the developer discharged.
Laid-Open Publication No. 4-118675 mentioned earlier teaches an arrangement including a developer replenishing chamber in which toner and a toner and carrier mixture are stored independently of each other, and a developer collecting chamber for collecting the deteriorated developer from a developer storing section included in a developing unit. When the toner concentration of the developer existing in the developer storing section decreases, as determined by a toner concentration sensor, the toner and the mixture are replenished into the storing section until the developer in the storing section reaches a preselected amount. Subsequently, the deteriorated developer is collected in the collecting chamber by an amount substantially equal to the total amount of the toner and mixture replenished. In this manner, the developer is replaced in a small amount at a time.
When the developer is replaced little by little before its life expires, as stated above, the developer in the developing unit is prevented from bodily reaching its life and maintains a stable degree of fatigue.
However, Laid-Open Publication No. 4-29271 pertains only to the replenishment and collection of a developer, so that toner must be replenished alone by some additional implementation. Specifically, the developer is automatically fed into the developing unit and collected therefrom. However, when the developer tank is replenished with a developer or replaced with a new developer tank, the operation is troublesome because the replenishing cycle of the developer is different from that of toner. Moreover, replenishment and collection or replacement is required with each of the developer tank, collecting container, and toner container, failing to free the serviceman or the operator from heavy burden. In addition, to reduce the time and labor for developer replacement, the developer tank and collecting container must be increased in size, rendering the entire apparatus bulky.
Assume that the developer is repeatedly replaced little by little, as taught in Laid-Open Publication Nos. 4-29271 and 4-118675. The prerequisite with this scheme is that to further stabilize image quality, not only the toner concentration of the developer depending on the toner consumption be controlled, but also the amount of the developer in the developing unit be confined in a certain range. Therefore, it is necessary to collect the carrier in a constant amount; otherwise, the amount of the developer would change due to repeated replacement. Laid-Open Publication No 4-29271 determines the consumption of the developer in terms of the number of printings produced or the duration of operation of the developing unit. This does not give any consideration to the consumption of the toner up to the time of replacement of the developer (which depends on the amount of image information), i.e., the toner concentration of the developer at the time of replacement.
Japanese Patent Laid-Open Publication 6-27809 pays attention to the fact that the deterioration of the developer is proportional to the duration of rotation of a rotary mechanism included in a developing unit. In accordance with this document, the duration of rotation of the rotary mechanism is counted and added up. Every time the cumulative duration reaches a preselected duration, fresh carrier or developer is replenished in a preselected amount such that a cumulative amount proportional to the cumulative duration is set up. Even this kind of approach determines the deterioration of the developer in terms of the duration of rotation, i.e., the duration of operation of the developing unit. On the other hand, for the collection of the developer, use is made of an overflow scheme. Because the overflow scheme does not take account of the toner concentration at the time of replacement of the developer, the amount of collection of the carrier is dependent on the toner concentration despite that the developer is replaced by a preselected amount. Although toner may be replenished alone later in order to adjust the toner concentration, the amount of the carrier in the developing unit is not corrected, causing the amount of the developer to change due to repeated replacement.
None of the prior art approaches described above gives consideration to problems particular to the initial installation of the apparatus at the user's station. Assume that the developing unit is loaded with the developer before the apparatus is transported to the user's station and operated for the first time there. Then, it is likely that the developer locally concentrates in the developing unit or drops via a gap between the developing unit and an image carrier. Therefore, at the time of forwarding, the developing unit, whether it be of the collective replacement type or of the little-by-little replacement type, must be held empty and then loaded with the developer from a container before the initial operation. Alternatively, the apparatus must include an exclusive section for storing a developer for the initial operation and automatically feed the developer to the developing unit before the initial operation. The former scheme needs the serviceman's or user's manual work while the latter makes the apparatus bulky and sophisticated due to the additional space for the developer. Even when the developer is replaced little by little, as taught in Laid-Open Publication No. 4-118675, the developer for the initial operation and to be fed from the exclusive chamber requires the chamber to have a considerable volume for the first loading of the developer, as in the collective replacement system. This also increases the overall size of the apparatus.
On the other hand, Japanese Patent Laid-Open Publication Nos. 60-159769 and 7-20705 each disclose a bottle for replenishing fresh toner into the developing unit. The bottle has a spiral ridge on its inner circumferential surface. When the bottle is rotated in its horizontal position, the spiral ridge drives the toner out of the bottle. However, the bottle is simply used to replenish the toner into the developing unit.
Further, Japanese Patent Laid-Open Publication No. 2-6978 proposes a toner container removable from the developing unit and consisting of a toner replenishing portion and a toner collecting portion. The container collects toner in the collecting portion separated from the replenishing portion. The collecting portion occupies an exclusive space in the container, and therefore increases the overall size of the apparatus. In addition, such a container is wasteful as to the space for stock.
In the above circumstances, the automatic collection of the deteriorated or used developer is essential which does not increase the size of the apparatus or trouble a serviceman or the user. As for the effective use of a limited space, it is preferable that the toner container storing fresh toner collects the used developer therein. However, the toner container includes means for promoting the efficient discharge of the toner, e.g., the spiral ridge taught in Laid-Open Publication No. 60-159769 or a scoop portion taught in Laid-Open Publication No. 7-20705. The scoop portion is implemented by a part of the inner surface of the shoulder portion of the container. Specifically, the inner surface with a greater diameter than a mouth portion bulges toward the edge of the mouth portion. However, such means for the effective discharge of the toner is undesirable when it comes to the collection of toner, because it prevents the expected amount of toner from being collected.
Japanese Patent Laid-Open Publication No. 7-171157, for example, teaches a bottle consisting of an annular first container and a hollow cylindrical second container received in the central through bore of the first container concentrically therewith. The two containers have concentric openings at the same side. A problem with this combined container scheme is that it must be rotated in opposite directions, increasing the cost of a drive source and the cost in the control aspect. Another problem is that the drive source needs an exclusive space and thereby increases the overall size of the apparatus.